Agricultural and construction machines often have to travel over very uneven terrain. In these cases, the driver is exposed to strong oscillations and impacts, which act on the driver via the bodywork and the seat. To reduce the vibrational load on the driver, the seats are generally equipped with passive suspension systems. Typically, pneumatic springs and hydraulic dampers are used for this purpose, and, together with a guide, connect the seat to the bodywork and reduce the transmission of vertical accelerations. However, these passive suspension systems rapidly reach the physical limit thereof, in particular for low-frequency oscillations. In this frequency range, the transmission of the disruptions is barely reduced and in some cases even increased.
In these conditions, a major improvement to the driver's comfort can only be achieved using an active suspension system. In active systems, various control elements are generally used instead of the passive elements such as springs and dampers. In known active suspension systems for agricultural and construction machines, hydraulic or electrical control elements are used to reduce the disruptive vertical accelerations on the driver from the vehicle by intervening in the movement of the seat.
The known hydraulic control elements consist of conventional double-action cylinders having a connected control unit for metering the hydraulic power required during the travel. The total power required for operating the active system is composed of the static component for supporting the driver and the dynamic component for actively reducing the transmissiveness of the suspension system. Because of the very high energy requirement, the necessary hydraulic power is provided outside the suspension system. The hydraulic control elements therefore always have to be connected to the adapted hydraulic on-board power system of the vehicle. The use of systems of this type in vehicles not equipped in this manner is thus greatly limited or impeded. In addition, this requires the use of the corresponding high-pressure lines having relatively narrow cross sections inside and outside the control element, which cause a strong development of noise because of the continuous flow of the hydraulic fluid through the system.
In most cases, the known electrical control elements consist of a conventional linear drive or of a conventional electric motor having a downstream transmission. The linear drives may be placed in the seat guide or between the seat base and the seat surface. The motor/transmission combinations are installed in the middle of the scissor mechanism often used as the seat guide. By comparison with hydraulic control elements, electrical control elements are much more energy-efficient because of the much higher efficiency. In the installation situation being described in the suspension system, the required total electrical power thereof is about half of the hydraulic power.
However, in control elements of motor/transmission combinations, the required total electrical power is somewhat higher because of the reduction in efficiency caused by the use of a strongly reducing transmission. In addition, the required electrical power is highly dependent on the vertical position of the seat.
In all known electrical control elements, relatively high electric currents occur in active operation, and have to be provided or discharged accordingly during rapid acceleration or braking of the load on the seat surface. To achieve a significant improvement in the oscillation insulation of the driver in active operation by comparison with passive operation, the electrical circuits of the control element thus have to be highly oversized. In addition, circuits of this type require sufficient ventilation of the suspension system so as to be able to keep the control elements at an acceptable operating temperature during the described rapid load change processes.
A further drawback of the above-described hydraulic and electrical active suspension systems is the lack of robustness if the supply of hydraulic or electrical power required for operation is interrupted or switched off. Without passive elements connected in parallel, these systems have no oscillation-reducing effect when switched off, and thus have no failsafe functionality. This can put the driver's safety at risk.
Therefore, in some known active systems, the electrical control elements are installed in the suspension system in parallel with the passive elements. This does provide failsafe functionality of the system, but in active operation the control elements have to apply an additional power component to overcome the forces of the passive elements.